Try using Natu. Natu has Magic Bounce, so while Whimsicott can use Substitue and all that, Whimsicott can't stall Natu at all. No Leech Seed, no Toxic, etc.
Natu @ Eviolite
Trait: Magic Bounce
EVs: 252 HP / 248 Def / 8 SpD
Calm Nature (+SpD, -Atk)
- Light Screen
- Reflect
- Roost/Toxic
- Psychic
This gives Natu a chance to set up Dual Screens while Whimsicott can only watch. roost for healing, or Toxic for stalling. Thunder Wave could also work. Psyhic is so do some damage in the case of Taunt.
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Hope This Helped :D